Fluidized bed combustion reactors are well known. These arrangements include a furnace section in which air is passed through a bed of particulate material, including a fossil fuel, such as coal, and a sulfur adsorbent, such as limestone, to fluidize the bed and to promote the combustion of the fuel at relatively low temperatures. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, the fluidized bed reactor offers an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed reactor includes what is commonly referred to as a bubbling fluidized bed in which a bed of particulate material is supported by an air distribution plate, to which combustion supporting air is introduced through a plurality of perforations in the plate, causing the material to expand and take on a suspended, or fluidized, state. The hot flue gases produced by the combustion of the fuel are passed to a heat recovery area to utilize their energy. Since the heat recovery area is usually separated from the furnace section, numerous expansion joints are required to connect the heat recovery area to the reactor in order to reduce stresses caused by the high temperature differentials. Heat losses are also encountered.
In the event the reactor is in the form of a steam generator, the walls of the reactor are formed by a plurality of heat transfer tubes. The heat produced by combustion within the fluidized bed is transferred to a heat exchange medium, such as water, circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, for separating the steam thus formed which steam is then combined with the steam produced in the heat recovery area and routed to a steam user or to a turbine to generate electricity.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn-down afforded by the bubbling bed, a circulating fluidized bed reactor has been developed utilizing a highly expanded and elutriating fluidized bed. According to this technique, the fluidized bed density may be below that of a typical bubbling fluidized bed, with the air velocity equal to or greater than that of a bubbling bed. The formation of the low density elutriating fluidized bed is due to its small particle size and to a high solids throughput, a result of the flue gases entraining the fine particulate solids. This high solids throughput requires greater solids recycling which is achieved by disposing a separator at the furnace section outlet to receive the flue gases, and the solids entrained therein, from the fluidized bed. The solids are separated from the flue gases in the separator and the flue gases are passed to a heat recovery area while the solids are recycled back to the furnace.
The high solids circulation required by the circulating fluidized bed makes it insensitive to fuel heat release patterns, thus minimizing the variation of the temperature within the reactor, and therefore decreasing the formation of nitrogen oxides. Also, this high solids recycling improves the efficiency of the separator. The resulting increase in sulfur adsorbent and fuel residence times reduces the consumption of adsorbent and fuel. Furthermore, the circulating fluidized bed inherently has more turn-down capability than the bubbling fluidized bed.
U.S. Pat. Nos. 4,809,623 and 4,809,625, assigned to the same assignee as the present application, disclose a fluidized bed reactor in which a dense, or bubbling, fluidized bed is maintained in the lower portion of the furnace section, while the bed is otherwise operated as a circulating fluidized bed. This "hybrid" design is such that advantages of both a bubbling bed and a circulating bed are obtained, not the least significant advantage being the ability to utilize particulate fuel material extending over a greater range of particle sizes.
In the operation of these types of fluidized beds, and, more particularly, those of the circulating and hybrid types, there are several important considerations. For example, the flue gases and entrained solids must be maintained in the furnace section at a particular temperature (usually approximately 1600.degree. F.) consistent with proper sulfur capture by the adsorbent. As a result, the maximum heat capacity (head) of the flue gases passed to the heat recovery area and the maximum heat capacity of the separated solids recycled through the separator to the furnace section are limited. In a cycle requiring only superheat duty and no reheat duty, the heat content of the flue gases at the furnace section outlet is usually sufficient to provide the necessary heat for use in the heat recovery area of the steam generator downstream of the separator. Therefore, the heat content of the recycled solids is not needed.
However, in a steam generator using a circulating or hybrid fluidized bed with sulfur capture and a cycle that requires reheat duty as well as superheater duty, the existing heat available in the flue gases at the furnace section outlet is often not sufficient. At the same time, heat in the reactor separator recycle loop is in excess of the steam generator duty requirements. For such a cycle, the design must be such that the heat in the recycled solids be utilized before the solids are reintroduced to the furnace section.
To provide this extra heat capacity, a recycle heat exchanger is sometimes located between the separator solids outlet and the fluidized bed of the furnace section. The recycle heat exchanger includes heat exchange surfaces and receives the separated solids from the separator and functions to transfer heat from the solids to the heat exchange surfaces at relatively high heat transfer rates before the solids are reintroduced to the furnace section. The heat acquired by the heat exchange surfaces is then transferred to cooling circuits to supply reheat and/or superheat duty.
A recycle heat exchanger can offer an extra benefit if constructed to act as a pressure sealing device. Such a sealing device is required between the low pressure separator solids outlet and the higher pressure furnace section of the reactor to prevent solids backflow and furnace section pressure fluctuations from adversely affecting the operating characteristics of either the separator or the furnace section.
There are, however, some disadvantages associated with the use of recycle heat exchangers. For example, a dedicated structure must be employed to house the recycle heat exchanger which must be fully insulated and include a fluidization system. Further, the structure housing the recycle heat exchanger must be interconnected with the rest of the reactor by costly expansion seal assemblies. In addition, if the recycle heat exchanger is to be used as a pressure sealing device, complex and costly structures are required, usually comprising individual chambers, for accomplishing the sealing function and the heat removal function, as well as to allow the solids to bypass the heat exchange surfaces during start-up.
Besides sometimes requiring recycle heat exchangers, circulating or hybrid fluidized bed combustion reactors also require relatively large separators for the separation of the entrained solid particles from the flue gases and for the solids recycle. A cyclone separator is commonly used which includes a vertically oriented, cylindrical vortex chamber in which a central gas outlet pipe is disposed for carrying the separated gases upwardly, while the separated particles exit the separator through its base. These so-called vertical cyclone separators are substantial in size and eliminate the possibility of a compact system design which can be modularized and easily transported and erected. For larger combustion systems, several vertical cyclone separators are often required to provide adequate particle separation, which compound the size problem and, in addition, usually require complicated gas duct arrangements which reduce operating efficiency. These ducts also require substantial amounts of costly refractory insulation to minimize heat loses and expansion seal assemblies to reduce thermal stresses.
Other problems also exist with the use of vertical cyclone separators since they require costly and complex components to deliver the separated particles back to the reactor's fluidized bed. In the absence of a recycle heat exchanger which functions as a sealing device, a gravity chute or a pneumatic transport system is required which must include a sealing device such as a sealpot, a siphon seal or a "J" or "L" valve due to the pressure differential between the low pressure cyclone discharge and the high pressure furnace section. Expansion joints are also required to connect the separator to the chute or transport system to reduce stresses caused by the high temperature differentials experienced.
To eliminate many of the above mentioned problems, horizontal cyclone separators characterized by a horizontally-oriented vortex chamber have been constructed. Horizontal cyclone separators may be readily configured within the upper portion of the furnace section and integrated with the walls of the furnace. However, known horizontal cyclone separators have various shortcomings, particularly with providing recycle heat exchange with the separated solids before the solids are reintroduced to the furnace section.